For acquiring objects or surfaces, use is often made of methods which progressively scan and in the process capture the topography of a structure, such as of a building, for example. In this case, such a topography constitutes a continuous sequence of points which describes the surface of the object, or else a corresponding model or a description of the surface. One conventional approach is scanning by means of a laser scanner which in each case acquires the spatial position of a surface point by the distance to the targeted surface point being measured by means of the laser and this measurement being combined with the angle information of the laser emission. From this distance and angle information, the spatial position of the acquired point can be determined and the surface can be continuously measured. In many cases, in parallel with this purely geometrical acquisition of the surface, image capture by means of a camera is also carried out, which, besides the overall visual view, also provides further information, e.g. regarding the surface texture.
In this regard, WO 97/40342, for example, describes a method which captures a topography by means of scanner systems installed in a stationary manner. A fixed installation point is chosen for these systems and serves as a basis for a scanning process brought about by motors. The three-dimensional location information of the respective surface point can be derived via the distance to the measured point, the angular position at the time of the measurement and the known location of the scanning apparatus. In this case, the scanner systems are specifically designed for the task of topography acquisition and scan a surface by movement of the scanner system or by variation of the beam path.
Moreover, scanning functions can be integrated into various other devices as additional functions. WO 2004/036145 discloses, for example, a geodetic measuring device which emits a laser beam for distance measurement from its position within the acquired range. Such measuring devices can likewise be modified for acquiring surfaces in a scanning fashion, or be operated without modification. One example thereof is motorized theodolites or total stations.
Other methods use mobile systems which scan a structure to be acquired by means of a movement of the scanner system, or support or supplement the scanning. Such systems are particularly suitable for acquiring linear or linearly navigable structures such as, for example, track systems, roads, tunnel systems or airfields.
Such acquisition processes in the prior art provide images or topographical data which substantially represent the information about the spatial distribution or arrangement relationship of surface points. If appropriate, additionally captured images allow further information to be derived. The structure and the course of the surface can thus be reconstructed comparatively well. What is disadvantageous, however, is the lack of qualitative indications about the type and constitution of the surface, in particular with regard to the internal structure or composition. In this regard, images captured in parallel with the scanning usually allow the identification of different brightness values. Furthermore, EP 1 759 172 describes a scanner system and a method for acquiring surfaces in spectrally resolved form which provides for deriving surface properties from the information obtained thereby.
Such laser scanners according to the prior art enable a user to acquire large surfaces and objects with a relatively short time expenditure—depending on a desired point-to-point resolution—completely and, if appropriate, with additional object information, but the accuracy of the point coordinates which can be derived in this case does not satisfy the high geodetic accuracy standards as established for example for modern measuring devices, in particular for total stations or theodolites.
Modern total stations generally have a compact and integrated design, wherein coaxial distance measuring elements and also computing, control and storage units are usually present in a device. Depending on the expansion stage of the total station, motorization of the targeting or sighting device and—in the case of the use of retroreflectors (for instance an all-round prism) as target objects—means for automatic target seeking and tracking can additionally be integrated. As a human-machine interface, the total station can have an electronic display control unit—generally a microprocessor computing unit with electronic data storage means—with display and input means, e.g. a keyboard. The measurement data acquired in an electrical-sensor-based manner are fed to the display control unit, such that the position of the target point can be determined, optically displayed and stored by the display control unit. Total stations known from the prior art can furthermore have a radio data interface for setting up a radio link to external peripheral components such as e.g. a handheld data acquisition device, which can be designed, in particular, as a data logger or field computer.
For sighting or targeting the target point to be measured, geodetic measuring devices of the generic type have a telescopic sight, such as e.g. an optical telescope, as sighting device. The telescopic sight is generally rotatable about a vertical axis and about a horizontal tilting axis relative to a base of the measuring device, such that the telescopic sight can be aligned with the point to be measured by pivoting and tilting. Modern devices can have, in addition to the optical viewing channel, a camera for sighting with angular seconds accuracy, said camera being integrated into the telescopic sight and being aligned for example coaxially or in a parallel fashion. The images or image sequences that can be acquired in this case, in particular a live image, can be represented on the display of the display control unit and/or on a display of the peripheral device—such as e.g. the data logger—used for remote control. In this case, the optical system of the sighting device can have a manual focus—for example an adjusting screw for altering the position of a focusing optical system—or an autofocus, wherein the focus position is altered e.g. by servomotors. By way of example, such a sighting device of a geodetic measuring device is described in EP 2 219 011. Automatic focusing devices for telescopic sights of geodetic devices are known e.g. from DE 197 107 22, DE 199 267 06 or DE 199 495 80.
Since target objects (e.g. the plumb rods with target mark, such as an all-round prism, which are usually used for geodetic purposes) cannot be targeted sufficiently precisely with the naked eye on the basis of the sighting device despite the 30-fold optical magnification often provided (i.e. not conforming to geodetic accuracy requirements), conventional measuring devices in the meantime have as standard an automatic target tracking function for prisms serving as target reflector (ATR: “Automatic Target Recognition”). For this, a further separate ATR light source—e.g. a multimode fiber output, which emits optical radiation having a wavelength in the range of 850 nm—and a specific ATR detector (e.g. CCD or CMOS area sensor) sensitive to said wavelength are conventionally additionally integrated in the telescope. By way of example, EP 2 141 450 describes a measuring device having a function for automatically targeting a retroreflective target and having an automatic target tracking functionality.
With such modern measuring devices, the coordinates of appropriate target points can be determined with a very high geodetic precision. What is disadvantageous in this case, however, is that a large-area object measurement e.g. with a total station means a disproportionately high time expenditure compared with a measuring process of a laser scanner on the object.